This invention relates to light redirecting films and film systems for redirecting light from a light source toward a direction normal to the plane of the films.
Light redirecting films are thin transparent or translucent optical films or substrates that redistribute the light passing through the films such that the distribution of the light exiting the films is directed more normal to the surface of the films. Heretofore, light redirecting films were provided with prismatic grooves, lenticular grooves, or pyramids on the light exit surface of the films which changed the angle of the film/air interface for light rays exiting the films and caused the components of the incident light distribution traveling in a plane perpendicular to the refracting surfaces of the grooves to be redistributed in a direction more normal to the surface of the films. Such light redirecting films are used, for example, with liquid crystal displays, used in laptop computers, word processors, avionic displays, cell phones, PDAs and the like to make the displays brighter.
The light entrance surface of the films usually has a transparent or matte finish depending on the visual appearance desired. A matte finish produces a softer image but is not as bright due to the additional scattering and resultant light loss caused by the matte or diffuse surface.
Heretofore, most applications used two grooved film layers rotated relative to each other such that the grooves in the respective film layers are at 90 degrees relative to each other. The reason for this is that a grooved light redirecting film will only redistribute, towards the direction normal to the film surface, the components of the incident light distribution traveling in a plane perpendicular to the refracting surfaces of the grooves. Therefore, to redirect light toward the normal of the film surface in two dimensions, two grooved film layers rotated 90 degrees with respect to each other are needed, one film layer to redirect light traveling in a plane perpendicular to the direction of its grooves and the other film layer to redirect light traveling in a plane perpendicular to the direction of its grooves.
Attempts have been made in the past to create a single layer light redirecting film that will redirect components of the incident light distribution traveling along two different axes 90 degrees to each other. One known way of accomplishing this is to provide a single layer film with two sets of grooves extending perpendicular to each other resulting in a pyramid structure which redirects light traveling in both such directions. However, such a film produces a much lower brightness than two film layers each with a single groove configuration rotated 90 degrees with respect to each other because the area that is removed from the first set of grooves by the second set of grooves in a single layer film reduces the surface area available to redirect light substantially by 50% in each direction of travel.
In addition, heretofore, the grooves of light redirecting films have been constructed so that all of the grooves meet the surface of the films at the same angle, mostly 45 degrees. This design assumes a constant, diffuse angular distribution of light from the light source, such as a lambertian source, a backlighting panel using a printing or etching technology to extract light, or a backlighting panel behind heavy diffusers. A light redirecting film where all of the light redirecting surfaces meet the film at the same angle is not optimized for a light source that has a nonuniform directional component to its light emission at different areas above the source. For example, the average angle about which a modern high efficiency edge lit backlight, using grooves or micro-optical surfaces to extract light, changes at different distances from the light source, requiring a different angle between the light redirecting surfaces and the plane of the film to optimally redirect light toward the normal of the film.
There is thus a need for a light redirecting film that can produce a softer image while eliminating the decrease in brightness associated with a matte or diffuse finish on the light input side of the film. Also, there is a need for a single layer of film which can redirect a portion of the light traveling in a plane parallel to the refracting surfaces in a grooved film, that would be brighter than a single layer of film using prismatic or lenticular grooves. In addition, there is a need for a light redirecting film that can compensate for the different angular distributions of light that may exist for a particular light source at different positions above the source, such as backlights used to illuminate liquid crystal displays. Also, there is a need for a light redirecting film system in which the film is matched or tuned to the light output distribution of a backlight or other light source to reorient or redirect more of the incident light from the backlight within a desired viewing angle.
The present invention relates to light redirecting films and light redirecting film systems that redistribute more of the light emitted by a backlight or other light source toward a direction more normal to the plane of the films, and to light redirecting films that produce a softer image without the brightness decrease associated with films that have a matte or diffuse finish on the light entrance surface of the films, for increased effectiveness.
The light exit surface of the films has a pattern of discrete individual optical elements of well defined shape for refracting the incident light distribution such that the distribution of light exiting the films is in a direction more normal to the surface of the films. These individual optical elements may be formed by depressions in or projections on the exit surface of the films, and include one or more sloping surfaces for refracting the incident light toward a direction normal to the exit surface. These sloping surfaces may for example include a combination of planar and curved surfaces that redirect the light within a desired viewing angle. Also, the curvature of the surfaces, or the ratio of the curved area to the planar area of the individual optical elements as well as the perimeter shapes of the curved and planar surfaces may be varied to tailor the light output distribution of the films, to customize the viewing angle of the display device used in conjunction with the films. In addition, the curvature of the surfaces, or the ratio of the curved area to the planar area of the individual optical elements may be varied to redirect more or less light that is traveling in a plane that would be parallel to the grooves of a prismatic or lenticular grooved film. Also the size and population of the individual optical elements, as well as the curvature of the surfaces of the individual optical elements may be chosen to produce a more or less diffuse output or to randomize the input light distribution from the light source to produce a softer more diffuse light output distribution while maintaining the output distribution within a specified angular region about the direction normal to the films.
The light entrance surface of the films may have an optical coating such as an antireflective coating, a reflective polarizer, a retardation coating or a polarizer. Also a matte or diffuse texture may be provided on the light entrance surface depending on the visual appearance desired. A matte finish produces a softer image but is not as bright.
The individual optical elements on the exit surface of the films may be randomized in such a way as to eliminate any interference with the pixel spacing of a liquid crystal display. This randomization can include the size, shape, position, depth, orientation, angle or density of the optical elements. This eliminates the need for diffuser layers to defeat moirxc3xa9 and similar effects. Also, at least some of the individual optical elements may be arranged in groupings across the exit surface of the films, with at least some of the optical elements in each of the groupings having a different size or shape characteristic that collectively produce an average size or shape characteristic for each of the groupings that varies across the films to obtain average characteristic values beyond machining tolerances for any single optical element and to defeat moirxc3xa9 and interference effects with the pixel spacing of a liquid crystal display. In addition, at least some of the individual optical elements may be oriented at different angles relative to each other for customizing the ability of the films to reorient/redirect light along two different axes.
The angles that the light redirecting surfaces of the individual optical elements make with the light exit surface of the films may also be varied across the display area of a liquid crystal display to tailor the light redirecting function of the films to a light input distribution that is non-uniform across the surface of the light source.
The individual optical elements of the light redirecting films also desirably overlap each other, in a staggered, interlocked and/or intersecting configuration, creating an optical structure with excellent surface area coverage. Moreover, the individual optical elements may be arranged in groupings with some of the individual optical elements oriented along one axis and other individual optical elements oriented along another axis. Also, the orientation of the individual optical elements in each grouping may vary. Further, the size, shape, position and/or orientation of the individual optical elements of the light redirecting films may vary to account for variations in the distribution of light emitted by a light source.
The properties and pattern of the optical elements of light redirecting films may also be customized to optimize the light redirecting films for different types of light sources which emit different light distributions, for example, one pattern for single bulb laptops, another pattern for double bulb flat panel displays, and so on.
Further, light redirecting film systems are provided in which the orientation, size, position and/or shape of the individual optical elements of the light redirecting films are tailored to the light output distribution of a backlight or other light source to reorient or redirect more of the incident light from the backlight within a desired viewing angle. Also, the backlight may include individual optical deformities that collimate light along one axis and the light redirecting films may include individual optical elements that collimate light along another axis perpendicular to the one axis.
To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter more fully described and particularly pointed out in the claims, the following description and annexed drawings setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but several of the various ways in which the principles of the invention may be employed.